1. Technical Field
The present disclosure relates to an acceleration and angular-velocity resonant detection integrated structure, and to a related sensor device of a so-called MEMS (microlectromechanical system) type.
2. Description of the Related Art
As is known, MEMS accelerometers and gyroscopes have been proposed and are used, thanks to their high compactness, their reduced levels of consumption, and their good electrical performance, in a wide range of contexts of application, for example in the field of portable electronic apparatuses, for inertial navigation applications, for creating user interfaces, or in general for detecting displacements in a three-dimensional space.
In particular, resonant micro-sensors have been proposed, made with the surface-micromachining technique, which base detection of external quantities on the variation of frequency of one or more elements set in resonance. Resonant detection, as compared to other measuring techniques, has the advantage of affording a direct frequency output, of a quasi-digital type, high sensitivity and a wide dynamic range.
In resonant accelerometers, the external acceleration to be measured produces a detectable shift of the resonance frequency of one or more resonator elements of the integrated mechanical detection structure. The resonator element may be constituted by an entire inertial mass (test mass or free mass, the so-called “proof mass”) of the integrated detection structure, by some part thereof, or by a distinct element, coupled to the inertial mass.
According to the configuration of the integrated detection structure, the variation of the resonance frequency may be induced by the presence, upon displacement of the inertial mass, of axial stresses in the resonator element, or of variations of the so-called “electrical stiffness” to which the same resonator element is subjected.
Resonant accelerometers, the operating principle of which is based on the detection of a variation of the resonance frequency due to axial stresses in the resonator element, are, for example, described in the following documents:    D. W. Bruns, R. D. Horning, W. R. Herb, J. D. Zook, H. Guckel “Resonant microbeam accelerometers”, Proc. Transducers 95, Stockholm, Sweden, June 25-29, 659-662 (1995); and    R. Zhu, G. Zhang, G. Chen “A novel resonant accelerometer based on nanoelectromechanical oscillator”, Proc. MEMS 2010, Hong Kong, 440-443 (2010).
Resonant accelerometers, the operating principle of which is, instead, based upon the detection of a variation of the resonance frequency due to a variation of electrical stiffness, are described, for example, in the following documents:    B. Lee, C. Oh, S. Lee, Y. Oh, K. Chun, “A vacuum packaged differential resonant accelerometer using gap sensitive electrostatic stiffness changing effect”, Proc. MEMS 2000; and    H. C. Kim, S. Seok, I. Kim, S-D. Choi, K. Chun, “Inertial-grade out-of-plane and in-plane differential resonant silicon accelerometers (DRXLs)”, Proc. Transducers '05, Seoul, Korea, June 5-9, 172-175 (2005).
Moreover, in Patent No. IT 1 395 419 and in Italian patent application No. T02011A000782 filed on Aug. 31, 2011 (related to WO2013030798), in the name of the present Applicants, resonant accelerometers that are improved as regards the characteristics (in particular the sensitivity) and the reduced mechanical dimensions are described.
In gyroscopes, generally, an inertial mass is made to vibrate at the natural resonance frequency and the effect due to the Coriolis force that originates on one or more detection elements in the presence of an external angular velocity is measured.
In general, the detection is made by means of the capacitive technique, while there exist a few examples of microgyroscopes with resonant detection, amongst which may the following documents may be cited, as examples:    A. A. Seshia, R. T. Howe, S. Montague, “An integrated microelectromechanical resonant output gyroscope”, Proc. MEMS2002, 722-726 (2002);    J. Li, J. Fang, H. Dong, Y. Tao, “Structure design and fabrication of a novel dual-mass resonant output micromechanical gyroscope”, Microsyst. Technology, 16, 543-552.
In these documents, the Coriolis force generates axial stresses in resonator elements, which modify their resonance frequency accordingly, enabling detection of the angular velocity.